A reference voltage generating circuit 50 as shown in FIG. 4 is known as an example of a reference voltage generating circuit in the related art. The reference voltage generating circuit 50 is referred to as a bandgap reference voltage generating circuit, and includes: npn transistors Q1, Q2 whose bases are commonly connected to an output terminal 1; current-mirror-connected pnp transistors Q3, Q4 that are connected as active loads to collectors of the transistors Q1, Q2; a pnp transistor Q5 whose base is connected to the collector of the transistor Q1, collector is connected to the output terminal 1 and the bases of the transistors Q1, Q2, and the emitter of the transistor Q5 is connected to a power supply terminal; and resistors R1, R2 connected in series. The resistor R1 is connected between an emitter of the transistor Q1 and an emitter of the transistor Q2, and the resistor R2 is connected between the emitter of the transistor Q2 and the ground.
A reference voltage VBG output to the output terminal 1 is represented by the following formula when an area ratio of the transistors Q1, Q2 is Q1:Q2=n:1, an area ratio of the transistors Q3, Q4 is Q3:Q4=1:1, and a voltage between the base and the emitter of the transistor Q2 is Vbe2.
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                      VBG        =                              Vbe            ⁢                                                  ⁢            2                    +                      2            ×            Vt            ×                          ln              ⁡                              (                n                )                                      ×                                          R                ⁢                                                                  ⁢                2                                            R                ⁢                                                                  ⁢                1                                                                        (        1        )            
Here, Vt is a thermal voltage (=kT/q, k: Boltzmann's constant, T: absolute temperature, q: electron charge), and has a positive temperature coefficient of substantially 0.0086 mV/° C. The voltage Vbe2 between the base and the emitter of the bipolar transistor Q2 has a negative temperature coefficient of substantially 2 mV.
Therefore, it is possible to generate the reference voltage VBG stable to temperature changes by using the two types of temperature coefficients to cancel each other out and setting values of n, R1, and R2.
However, a voltage Vbe between a base and an emitter of a bipolar transistor actually has a slight second-order temperature coefficient. Accordingly, as shown in FIG. 5, a temperature characteristic of the reference voltage VBG has a second-order temperature dependence that the reference voltage VBG decreases from a room temperature region A to a high temperature region B and a low temperature region C. For this reason, the slight temperature dependence may be a problem in an application such as an automotive application, in which stability is required for a wide temperature range.
Patent Document 1 has proposed a method of cancelling such a second-order temperature characteristic.